Which retailers are abolishing sideloading? In terms of eink based ereaders, I've only used Amazon, Kobo, and Sony. All of them were sideloading friendly. I have a hard time seeing how the smaller vendors would be able to impose such a policy.

What I have seen are vendors that are hostile towards sideloading DRMed content from other sources. Amazon fits into that category, though my Kobo and Sony devices were more than happy to accept content that used Adobe's DRM scheme -- including library books, which you would think is their biggest competitor on the basis of cost.